An overview of several aspects of the weathering of roofing materials is presented. Degradation of materials initiated by ultraviolet radiation is discussed for plastics used in roofing, as well as wood and asphalt. Elevated temperatures accelerate many deleterious chemical reactions and hasten diffusion of material components. Effects of moisture include decay of wood, acceleration of corrosion of metals, staining of clay, and freeze-thaw damage. Soiling of roofing materials causes objectionable stains and reduces the solar reflectance of reflective materials. (Soiling of non-reflective materials can also increase solar reflectance.) Soiling can be attributed to biological growth (e.g., cyanobacteria, fungi, algae), deposits of organic and mineral particles, and to the accumulation of fly ash, hydrocarbons and soot from combustion.

We describe methods for creating solar-reflective nonwhite surfaces and their application to a wide variety of residential roofing materials, including metal, clay tile, concrete tile, wood, and asphalt shingle. Reflectance in the near-infrared (NIR) spectrum (0.7-2.5 μm) is maximized by coloring a topcoat with pigments that weakly absorb and (optionally) strongly backscatter NIR radiation, and by adding an NIR-reflective basecoat (e.g., one colored with titanium dioxide rutile white) if both the topcoat and the substrate weakly reflect NIR radiation. Coated steel and glazed clay-tile roofing products achieved NIR reflectances of up to 0.50 and 0.75, respectively, using only cool topcoats. Gray-cement concrete tiles achieved NIR reflectances as high as 0.60 with coatings colored by NIR-scattering pigments. Such tiles could attain NIR reflectances of up to 0.85 by overlaying a white basecoat with a topcoat colored by NIR-transparent organic pigments. Granule-surfaced asphalt shingles achieved NIR reflectances as high as 0.45 when the granules were covered with a white basecoat and a cool color topcoat.

Raising roof reflectivity from an existing 10-20% to about 60% can reduce cooling-energy use in buildings in excess of 20%. Cool roofs also result in a lower ambient temperature that further decreases the need for air conditioning and retards smog formation. In 2002, suitable cool white materials were available for most roof products, with the notable exception of asphalt shingles; cooler colored materials are needed for all types of roofing. To help to fill this gap, the California Energy Commission (Energy Commission) engaged Lawrence Berkeley National Laboratory (LBNL) and Oak Ridge National Laboratory (ORNL) to work on a three-year project with the roofing industry to develop and produce reflective, colored roofing products. The intended outcome of this project was to make cool-colored roofing materials a market reality within three to five years. For residential shingles, we have developed prototype light-colored shingles with solar reflectances of up to 35%. One manufacturer currently markets colored shingles with the ENERGY STAR qualifying solar reflectance of 0.25. Colored metal and clay tile roofing materials with solar reflectances of 0.30 to 0.60 are currently available in theCalifornia market.

LBNL and ORNL performed research & development in conjunction with pigment manufacturers, and worked with roofing materials manufacturers to reduce the sunlit temperatures of nonwhite asphalt shingles, clay tiles, concrete tiles, metal products, and wood shakes. A significant portion of the effort was devoted to identification and characterization of pigments to include and exclude in cool coating systems, and to the development of engineering methods for effective and economic incorporation of cool pigments in roofing materials. The project also measured and documented the laboratory and in-situ performances of roofing products. We also established and monitored three pairs of demonstration homes to measure and showcase the energy-saving benefits of cool roofs. The following activities were carried out.

In collaboration with the Energy Commission, we convened a Project Advisory Committee (PAC), composed of 15 to 20 diverse professionals, to provide strategic guidance to the project.

In order to determine how to optimize the solar reflectance of a pigmented coating matching a particular color, and how the performance of cool-colored roofing products compares to that of a standard material, we (1) measured and characterized the optical properties of many standard and innovative pigmentation materials; (2) developed a computer model to maximize the solar reflectance of roofing materials for a choice of visible colors; and (3) created a database ofcharacteristics of cool pigments.

In order to help manufacturers design innovative methods to produce cool-colored roofing materials, we (1) compiled information on roofing materials manufacturing methods; (2) worked with roofing manufacturers to design innovative production methods for cool-colored materials; and (3) tested the performance of materials in weather-testing facilities.

One of the project objectives was to demonstrate, measure and document the building energy savings, improved durability and sustainability attained by use of cool-colored roof materials to key stakeholders (consumers, roofing manufacturers, roofing contractors, and retail home improvement centers). In order to do this, we (1) monitored buildings at California demonstration sites to measure and document the energy savings of cool-colored roof materials; (2) conducted materials testing at weathering farms in California; (3) conducted thermal testing at the ORNL Steep-slope Assembly Testing Facility; and (4) performed a detailed study toinvestigate the effect of solar reflectance on product useful life.

We developed partnerships with various members of the roofing industry. We worked through the trade associations to communicate and advertise to their membership new cool color roof technology and products. This collaboration induced the manufacturers to develop a market plan for Ca ifornia and to provide technical input and support for this activity. Through the industry partners, many California housing developers and contractors have been convinced to install the new cool-colored roofing products.

Urban areas tend to have higher air temperatures than their rural surroundings as a result of gradual surface modifications that include replacing the natural vegetation with buildings and roads. The term "Urban Heat Island" describes this phenomenon. The surfaces of buildings and pavements absorb solar radiation and become extremely hot, which in turn warm the surrounding air. Cities that have been "paved over" do not receive the benefit of the natural cooling effect of vegetation. As the air temperature rises, so does the demand for air-conditioning (a/c). This leads to higher emissions from power plants, as well as increased smog formation as a result of warmer temperatures. In the United States, we have found that this increase in air temperature is responsible for 5–10% of urban peak electric demand for a/c use, and as much as 20% of population weighted smog concentrations in urban areas.

Simple ways to cool the cities are the use of reflective surfaces (rooftops and pavements) and planting of urban vegetation. On a large scale, the evapotranspiration from vegetation and increased reflection of incoming solar radiation by reflective surfaces will cool a community a few degrees in the summer. As an example, computer simulations for Los Angeles, CA show that resurfacing about two-third of the pavements and rooftops with reflective surfaces and planting three trees per house can cool down LA by an average of 2–3K. This reduction in air temperature will reduce urban smog exposure in the LA basin by roughly the same amount as removing the basin entire onroad vehicle exhaust. Heat island mitigation is an effective air pollution control strategy, more than paying for itself in cooling energy cost savings. We estimate that the cooling energy savings in U.S. from cool surfaces and shade trees, when fully implemented, is about $5 billion per year (about $100 per air-conditioned house).

Raising the solar reflectance of a roof from a typical value of 0.1 – 0.2 to an achievable 0.6 can reduce cooling-energy use in buildings by more than 20%. Cool roofs also reduce ambient outside air temperature, thus further decreasing the need for air conditioning and retarding smog formation.

We are collaborating with pigment manufacturers to characterize colorants, and with manufacturers of roofing materials to produce cool colored products, including asphalt shingles, tiles, metal roofing, wood shakes, membranes, and coatings. Significant efforts are being devoted to the identification and characterization of pigments suitable for cool-colored coatings, and to the development of engineering methods for applying cool coatings to roofing materials. We are also measuring and documenting the laboratory and in-situ performances of roofing products. Demonstration of energy savings can accelerate the market penetration of cool-colored roofing materials. Early results from this program have yielded colored concrete, clay, and metal roofing products with solar reflectances exceeding 0.4. Obtaining equally high reflectances for roofing shingles is more challenging, but we expect manufacturers to soon have several cost-effective colored shingles with reflectances of at least 0.25.